Silver halide photographic elements contain light sensitive silver halide in a hydrophilic emulsion. An image is formed in the element by exposing the silver halide to light, or to other actinic radiation, and developing the exposed silver halide to reduce it to elemental silver.
In color photographic elements a dye image is formed as a consequence of silver halide development by one of several different processes. The most common is to allow a by-product of silver halide development, oxidized silver halide developing agent, to react with a dye forming compound called a coupler. The silver and unreacted silver halide are then removed from the photographic element, leaving a dye image.
In either case, formation of the image commonly involves liquid processing with aqueous solutions that must penetrate the surface of the element to come into contact with silver halide and coupler. Thus, gelatin, and similar natural or synthetic hydrophilic polymers, have proven to be the binders of choice for silver halide photographic elements. Unfortunately, when gelatin, and similar polymers, are formulated so as to facilitate contact between the silver halide crystal and aqueous processing solutions, they are not as tough and mar-resistant as would be desired for something that is handled in the way that an imaged photographic element may be handled. Thus, the imaged element can be easily marked by fingerprints, it can be scratched or torn and it can swell or otherwise deform when it is contacted with liquids.
There have been attempts over the years to provide protective layers for gelatin based photographic systems that will protect the images from damages by water or aqueous solutions. U.S. Pat. No. 2,173,480 describes a method of applying a colloidal suspension to moist film as the last step of photographic processing before drying. A series of patents describes methods of solvent coating a protective layer on the image after photographic processing is completed and are described in U.S. Pat. Nos. 2,259,009, 2,331,746, 2,798,004, 3,113,867, 3,190,197, 3,415,670 and 3,733,293. The application of UV-polymerizable monomers and oligomers on processed image followed by radiation exposure to form crosslinked protective layer is described U.S. Pat. Nos. 4,092,173, 4,171,979, 4,333,998 and 4,426,431. One drawback for the solvent coating method and the radiation cure method is the health and environmental concern of those chemicals to the coating operator. U.S. Pat. Nos. 3,397,980, 3,697,277 and 4,999,266 describe methods of laminating polymeric sheet film on the processed image as the protective layer. U.S. Pat. No. 5,447,832 describes the use of a protective layer containing mixture of high and low Tg latices as the water-resistance layer to preserve the antistat property of the V(2) O(5) layer through photographic processing. This protective layer is not applicable to the image formation layers since it will detrimentally inhibit the photographic processing. U.S. Pat. No. 2,706,686 describes the formation of a lacquer finish for photographic emulsions, with the aim of providing water- and fingerprint-resistance by coating the emulsion, prior to exposure, with a porous layer that has a high degree of water permeability to the processing solutions. After processing, the lacquer layer is fused and coalesced into a continuous, impervious coating. The porous layer is achieved by coating a mixture of a lacquer and a solid removable extender (ammonium carbonate), and removing the extender by sublimation or dissolution during processing. The overcoat as described is coated as a suspension in an organic solvent, and thus is not desirable for large-scale application. U.S. Pat. No. 3,443,946 provides a roughened (matte) scratch-protective layer, but not a water-impermeable one. U.S. Pat. No. 3,502,501 provides protection against mechanical damage only; the layer in question contains a majority of hydrophilic polymeric materials, and must be permeable to water in order to maintain processability. U.S. Pat. No. 5,179,147 likewise provides a layer that is not water-protective.
U.S. Pat. No 5,856,051 describes an aqueous coatable, water-resistant protective overcoat that can be incorporated into the photographic product, allows for appropriate diffusion of photographic processing solutions, and does not require coating operation after exposure and processing. This was accomplished by applying a coating comprising hydrophobic polymer particles having an average size of 0.01 to 1 microns to the silver halide light-sensitive emulsion layer. The silver halide light sensitive emulsion layer is developed to provide an imaged photographic element. The hydrophobic polymer particles are then fused to form a protective overcoat. This patent did not however describe the composition of any suitable materials for fusing the hydrophobic polymer particles to form the protective layer.
One key requirement of the method for fusing the particles comprising the protective overcoat is that the desired gloss level of the original unprotected photographic element be maintained. In the field of electrophotography, belt fusers have been shown to yield images with gloss values comparable to photographic elements. The belt in the belt fusing system can be made of stainless steel or polyester and the outer surface of the fuser member can be aluminum, steel, various alloys, or polymeric materials, such as, thermoset resins and fluoroelastomers.
The background art of electrophotography discloses several broad classes of materials useful for fuser belts. For example, U.S. Pat. Nos. 5,089,363; 5,465,146; 5,386,281; 5,362,833; 5,529,847; 5,330,840; 5,233,008; 5,200,284 and 5,124,755 disclose fuser belt systems consisting of belts coated with silicone polymers. U.S. Pat. No. 5,089,363 discloses that metal belts coated with highly crosslinked polysiloxanes provide fused toner images having high gloss.
Commonly-assigned U.S. Pat. No. 5,804,341 describes an electrostatically bound water-resistant protective overcoat that can be attached into the finished photographic product. This was accomplished by electrostatically binding a coating comprising hydrophobic polymer particles having an average size of 3 to 10 microns on to the silver halide light-sensitive emulsion layer after silver halide light sensitive emulsion layer is developed to provide an imaged photographic element. The hydrophobic polymer particles are then fused to form a protective overcoat.
Through the recent advances in the development of protective overcoats for photographic elements; further materials are required to fuse the particulate polymers composing the protective overcoats described in U.S. Pat. Nos. 5,856,051 and 5,804,341. These materials will be in the form of overcoated fusing belts which provide high gloss, longlife, and good release of the fused heat-softenable polymers images.